The development of ultralow threshold micro- and nanoscale lasers is of critical importance for applications in future ultrahigh-speed photonic systems as well as quantum information processing. In fact, a critical yet missing technology for future chip-level optical communications is a high performance and highly reliable laser on silicon. In this regard, III-V semiconductor lasers on Si as well as the monolithic integration with Si-based waveguide devices have been extensively investigated. However, their practical applications have been limited, to a large extent, by the generation and propagation of dislocations, due to the large lattice and thermal mismatches between III-V materials and Si.
Accordingly, research has focused to reducing the dimensions of the optical devices to both reduce the effects of these large lattice and thermal mismatches with devices of reduced dimensions but to also provide optical devices that do not absorb large areas of silicon circuits. In recent years significant progress has been made in various semiconductor microcavities, such as microdisks, micropillars, and photonic crystals, wherein quantum wells or dots are often incorporated as the gain media. The realization of such devices, however, generally involves the use of fairly complicated top-down fabrication processes and to date has been limited to devices that operate at low temperatures and/or pulsed modes. Recently, rolled-up micro- and nanotube based optical cavities have been intensively investigated, which are formed when coherently strained semiconductor heterostructures are selectively released from the host substrates. Combining the advantages of both top-down and bottom-up fabrication processes, this approach provides a great degree of flexibility in tailoring the optical modes by varying the tube diameters, wall thicknesses, as well as the surface geometry. The resulting micro and nanotube cavities can exhibit epitaxially smooth surface and a near-perfect overlap between the maximum optical field intensity and the gain medium, which are well suited for realizing lasers with low threshold.
Since the first demonstration by Prinz et al., rolled-up semiconductor micro- and nanotubes have been realized utilizing InGaAs/GaAs, SiGe, and SiO/SiO2 based materials and have been investigated for applications in nanophotonic devices and biosensors. Optical resonance modes in rolled-up InAs/GaAs semiconductor tube structures were first demonstrated at very low temperatures of approximately 5 K by Kipp et al and their unique mode profiles were also analyzed. Subsequently, coherent emission from rolled-up InGaAs/GaAs quantum dot semiconductor tubes at 77 K and room temperature has been demonstrated. More recently, room-temperature lasing has been achieved from free-standing InGaAs/GaAs quantum dot semiconductor tube optical ring resonators under optical pumping. With the development of the special transfer techniques, including substrate-on-substrate and fiber taper assisted transfer processes, nearly defect-free rolled-up semiconductor tube devices can be readily achieved on Si or any other foreign substrates, thereby providing a highly promising approach for realizing high performance CMOS compatible nanoscale lasers.
For practical applications, electrically injected micro- and nanoscale lasers are required. Although electrically injected microdisk, microcylinder, and photonic crystal based devices have been extensively investigated, the achievement of high efficiency, highly reliable, and low threshold operation has remained challenging. In this regard, rolled-up micro and nanotubes provide several important advantages. The epitaxially smooth surface can greatly reduce carrier nonradiative recombination associated with the presence of surface states. In addition, the relative large surface area of a rolled-up semiconductor tube ring resonator makes it possible to place the electrical contacts in the vicinity of the laser active region without adversely affecting the optical mode profiles and device performance. To date, electrically injected rolled-up semiconductor tube based devices have not been reported and it would be beneficial therefore to provide a means of manufacturing such devices.
Within these micro- and nanoscale optical devices it has been envisioned that the incorporation of quantum dots in high Q optical microcavities would lead to nanoscale lasers with potentially ultralow or near-zero threshold, temperature invariant operation, and ultrahigh-speed frequency response, providing the basic building blocks for chip-level optical communications and future quantum networking systems. It would therefore be further beneficial for such microcavities to be manufactured with high quality self-organized quantum dot heterostructures with near-discrete density of states providing large and broad gain. Accordingly the provisioning of electrical injected optical semiconductor tubes would benefit from incorporating strained heteroepitaxy growth to provide such self-organized quantum dot arrays that will be required in providing semiconductor tubes that are initially manufactured using planar semiconductor processing and then rolled-up for compatibility with standard semiconductor processing techniques.
It would also be evident that in many applications direct growth of the optical semiconductor tubes would either not be feasible through processing incompatibilities, performance requirements, etc or that the substrate area required for their fabrication that is then not exploited in the final device. As such the controlled transfer and exact positioning of micro- or nanoscale lasers on a processed complementary metal-oxide-semiconductor (CMOS) chip and their subsequent integration with Si bus waveguides and modulators is in demand for next-generation chip-level optical interconnects. In spite of the significant progress made in dry printing, wafer bonding, solution casting, and more recently substrate-on-substrate (SOS) transfer processes, the reliable transfer of a high-quality optical microcavity on a foreign substrate has hitherto not been achieved. These techniques have been limited, to a large extent, by the unique properties of these optical micro- and nanocavities, which generally rely on the use of free-standing nanomembranes. Consequently, the commonly used dry printing and/or stamping techniques may significantly alter the structural and optical properties of the devices. The presence of a large surface tension for conventional photonic crystal, microdisk, and micropillar devices also makes it extremely difficult to detach the cavities from the handling substrate.
Rolled-up semiconductor micro- and nanotubes, formed when a coherently strained semiconductor bilayer is selectively released from the host substrate can be controllably released from the handling substrate with low stress/strain by selectively etching the underlying sacrificial layer completely beneath the micro- or nanotubes. SOS transfer and solution casting could also used to realize such devices on Si or other foreign substrates. However, a controlled transfer and exact positioning of single micro- and nanotube devices has not been possible with these techniques. In this context, it would be beneficial to have a highly accurate and flexible technique for transferring single rolled-up micro- and nanotube devices that have diameters of a few microns, wall thicknesses of a few nanometers and lengths of several hundreds of microns. It would therefore be beneficial for the micro- and nanotubes to be picked up from their handling substrates and transferred, with a precisely controlled position, directly on the facet of a single-mode fiber. Such an approach offering greatly simplified packaging of the optical devices within the context of silicon microelectronics.
It is, therefore, desirable to provide a means of manufacturing micro- and nanotubes with high quality allowing the incorporation of ordered structures such as quantum wells and quantum dots and for the method to allow in some applications for high densities of micro- and nanotubes. It would be further beneficial for the method to provide a means of releasing these micro- and nanotubes with low stress and was compatible with a method of “pick-and-place” allowing micro- and nanotubes to be exploited in devices integrated on substrates that are either incompatible with the manufacturing technique or where the area of substrate required to manufacture them is detrimental to the cost or performance of the circuit. It would also therefore be beneficial to provide a means of “pick-and-place” that is low stress.